(1) Field of the Invention
The present invention relates to means for influencing the flow of air over the outside surfaces of rotorcraft. The present invention relates more particularly to a fairing conventionally arranged at the top of a rotorcraft being located under the rotary wing of a main rotor having substantially a vertical axis and above a cover covering a power plant of the rotorcraft.
(2) Description of Related Art
A rotorcraft is a rotary wing aircraft having one or more rotors, including at least a main rotor of substantially vertical axis arranged at the top of the rotorcraft. The main rotor provides the rotorcraft at least with lift, and in the specific circumstance of a helicopter, possibly also with propulsion and/or guidance along any movement axis.
A rotorcraft may also have at least one auxiliary rotor of substantially horizontal axis. Conventionally, such an auxiliary rotor is installed at the end of a tail boom in order to control the yaw attitude of the rotorcraft. Such an auxiliary rotor can also be used to provide a rotorcraft with a propulsive propeller for moving the rotorcraft in translation.
The rotor(s) of a rotorcraft is/are conventionally driven in rotation by a power plant including at least one fuel-burning engine, in particular a turboshaft engine. The power plant is typically located in the upper portion of the rotorcraft beneath the rotary wing of the main rotor. A cover is arranged around the power plant in order to direct the flow of air towards the rear of the rotorcraft and enhance the aerodynamic characteristics of the rotorcraft.
Such a cover is commonly raised by a fairing that covers a mechanism for operating blades of the rotary wing of the main rotor in compliance with flight commands generated by a pilot of the rotorcraft.
For example, such a mechanism for controlling the blades of the rotary wing of the main rotor is commonly formed by a pair of “swashplates” movably mounted on a mast carrying the rotary wing. The swashplates are operated by a pilot of the rotorcraft using various control lines in order to cause the pitch of the blades of the rotary wing of the main rotor to vary collectively and/or cyclically so as to modify the attitude of the rotorcraft.
The cover and the fairing that raises it extend mainly in a longitudinal direction corresponding to the longitudinal direction in which the rotorcraft extends, conventionally considered as going between the front and the rear of the rotorcraft when on the ground.
Naturally, concepts such as “top”, “high portion”, “below”, “bottom”, or “raising”, for example are commonly understood relative to the vertical orientation of the rotorcraft when on the ground. Likewise, concepts such as “vertical plane” and “horizontal plane” of the rotorcraft are commonly understood respectively relative to the vertical and horizontal general directions in which the rotorcraft extends when on the ground.
In this context, it is appropriate to arrange the fairing so as to reduce its aerodynamic drag as much as possible and so as to limit the wake that it generates. The arrangement of the fairing must in particular encourage the stream of air to remain attached along its surface by acting on the turbulence phenomena occurring at the rear of the fairing. Turbulent vortices form at the rear of the fairing where they constitute a source of drag and give rise to structural excitation of the outer walls of the rotorcraft and of the tail boom extending towards the rear of the rotorcraft.
A common solution consists in shaping the profile of the fairing in question to have the shape of a drop of water in a horizontal plane of the rotorcraft. In this context, reference may be made for example to the following documents: US 2011/036954 (Frederick W. Piasecki), U.S. Pat. No. 3,331,444 (Charles V. Toner), US 2012/104157 (Van Der Westhuizen), DE 1 114 394 (United Aircraft Corp.), and US 2006/269411 (Fabio P. Bertolotti).
It has nevertheless been found that such a solution is not fully satisfactory for optimizing stabilization of the rotorcraft in the event of a side wind, of flying sideways, and/or of the rotorcraft sideslipping in yaw, in particular for light rotorcraft where it is desired to enhance their maneuverability in flight.
More particularly, among the constraints on how the fairing should be arranged, the flying quality of the rotorcraft must be preserved. In particular, it is necessary to avoid the flow of air along the fairing giving rise to instabilities in maintaining a heading and/or to disturbances in yaw attitude control of the rotorcraft. The sound nuisance produced by the flow of air along the outside surfaces of the rotorcraft must also be limited as well as possible.
It is found that when the flow of air around the rotorcraft includes a significant sideways vector component, the regularity of the flow of said turbulence towards the rear of the fairing is disturbed, and consequently the stability of the attitude of the rotorcraft in yaw is affected thereby.
More particularly, the turbulent vortices generated at the rear end of the fairing tend to move along the tail boom at a frequency and in a flow direction that are random.
Such an instability in the flow of the turbulence, which is made worse in the event of a side wind, of flying sideways, and/or of the rotorcraft sideslipping in yaw, has the effect of producing unwelcome vibration that may harm the mechanical strength of the rotorcraft and that can degrade passenger comfort. Furthermore, such a situation also gives rise to an increase in sound nuisance and tends to destabilize the yaw behavior of the rotorcraft.
In order to mitigate those drawbacks, it is known to truncate the waterdrop-shaped profile of the fairing transversely at its rear end. The effect of this is to improve the regularity of the flow of the turbulence in the event of a side wind, of flying sideways, and/or of sideslipping in yaw. Nevertheless, arranging the trailing edge of the fairing in that way tends to increase its aerodynamic drag, which is undesirable.
Finally, it is found that various specific arrangements for shaping the fairing in order to improve the flow of air for certain flight situations are unsuitable for other flight situations. As a result, it is necessary to find a compromise between various arrangements for the fairing in order to obtain the best possible flow of air that is satisfactory regardless of the flight situation of the rotorcraft.
It is also known for the rear wall of the fairing that is formed by truncating its waterdrop-shaped profile to be arranged as a plane that slopes relative to the horizontal plane of the rotorcraft. Such provisions seek to enhance the guidance of the flow of air along the fairing towards the bottom of the rotorcraft, it being understood that the rotation of the main rotor itself generates a powerful flow of air directed towards the bottom of the rotorcraft together with complex aerodynamic phenomena that disturb the stability of the behavior of the rotorcraft in flight.
Consequently, it is desirable for the overall arrangement and for specific features of the fairing concerning the flow of air along its surfaces to take account also of the wash generated by the main rotor.
However minor they might be, the various features applied to the fairing for the purpose of influencing the flow of air along its surfaces, and consequently along members arranged to the rear of the rotorcraft, interfere in combination with one another on the overall conditions of such a flow and of its effects.
Consequently, choices need to be made concerning using the various fairing features singly or jointly in order to obtain a flow of air towards the rear of the rotorcraft that is at best given the diversity of the problems mentioned above.
It is also desirable for the arrangement of the fairing to be usable without major modification, regardless of the general configuration of the rotorcraft, in order to avoid preparing a specific shape for the fairing that depends on the general structure of the rotorcraft, which structure varies among various families of rotorcraft.
As a result, there is a continuing search for a fairing shape that enhances a flow of air along its outside surface to obtain results that are satisfactory at best concerning the various constraints and requirements mentioned above. Thus, the specific aerodynamic arrangements selected for the outside surfaces of the fairing need to be selected in isolation while taking account of the effects that are produced when they are combined with one another.
The present invention lies in the context of such a search and seeks to propose a rotorcraft having a fairing of the above-mentioned type arranged at the top of the rotorcraft and presenting overall arrangements and specific aerodynamic features that ensure a flow of air towards the rear of the rotorcraft that satisfies the various above-mentioned problems that are raised in a manner that is satisfactory.